i am trying to get the incomming serial message into msg. because msg is the variable wich will be sended over the RF.
There is NOTHING magic about the name msg. ANY character array that is properly NULL terminated, as inData would be, is suitable input to any function that expects a const char * variable.
What is sending the serial data to the Arduino? Do you have any control over that process? If so, you need to make that process send "<your data>". The < and the > define the start and end of the packet. The code I posted will store everything between the < and the > in inData, if there is room. You can then send inData using the radio sending code.